1. Field of the Invention
The present invention relates to a low yield point steel having excellent workability and non-aging properties, which can be effectively used as a plate (thick steel plate, hot rolled plate and cold rolled plate) or rod (wires, rods and bars) which is subjected to and/or severe forming.
2. Description of the Prior Art
In the process of preparing hot rolled steel plates, a hot rolled band-like steel plate is coiled to form a coil of the steel plate. This coil is marketed directly as a "coil material," after the coil is cut into plates of a certain length, the steel is marketed as a "cut plate material". The hot rolled steel which is marketed is further cut or subjected to forming whereby an article having a shape suitable for the intended object and use is obtained.
In the process of preparing steel rods, a hot rolled rod is subjected to cold drawing, and the resulting steel rod is marketed.
Most of these marketed hot rolled plates and rods or bars are subjected to cold forming to form articles having a shape suitable for the intended use of the article. In the case of hot rolled plates, this cold forming includes deep drawing, bending, bulging and shearing and combinations of these treatments. Rods are subjected to drawing. Accordingly, cold workability is of decisive significance for these steels. The typical characteristics which are indicative of the workability of steel plate are the yield point, elongation, n value (work hardening exponent), r value (Rankford value) and Erichsen value, and the foregoing steel plates must have proper or excellent values with respect to these test items. The property which rods must possess is the low breakage ratio even if the reduction ratio of the rod is high at the drawing step.
In order to improve the cold formability or cold drawability, of the steel a very low carbon aluminum killed steel is employed in the case of hot rolled steel plates, a very low carbon aluminum killed steel or very low carbon decarburized denitrided rimmed steel is employed in the case of cold rolled steel plates and a very low carbon rimmed steel is employed in the case of rods.
However, these steels are posses insufficient yield point characterists which is one of the characteristics which expresses the workability of the steel. In the case of a hot rolled plate of a very low carbon aluminum killed steel, it is very difficult to reduce the yield point below 21-22 Kg/mm.sup.2 even after temper rolling, and the lower limit of the yield point of a very low carbon decarburized dentrided rimmed steel is about 18 Kg/mm.sup.2.
"Hot rolled steel plate for deep drawing possessing low yield point and non-aging properties has previously been disclosed in" Japanese Patent Application No. 129074/74. As set forth in the claim of the above patent application, the invention relates to a hot rolled steel plate for deep drawing having the low yield point and a non-aging property of the as hot rolled state, which consists in weight percent of 0.03 to 0.07% C, 0.01 to 0.25% Si, 0.2 to 0.5% Mn, 0.015 to 0.07% of Al and 0.03 to 0.1% of Zr exclusive of Zr in its oxide and sulfide forms, the balance being iron and inevitable impurities. This steel currently on the market and is being put to practical use. Also in this steel plate, the lower limit of the yield point in the as hot rolled state is about 23 Kg/mm.sup.2 or higher. Even if such a hot rolled product is heat treated at 950.degree. C. for 1 hour, the lower limit of the yield point is as high as about 20 Kg/mm.sup.2.
An age-hardening cold rolled steel plate for deep drawing, which is excellent in the deep drawability, bulging characteristics and other pressformability characteristics and in which a high strength can be imparted by an age-hardening treatment after forming, and the method for preparing such a cold rolled steel are disclosed in the specification of Japanese Patent Application No. 60276/70 (Japanese Patent Publication No. 17013/75.) As set forth in the claims of this application, the application provides (a) an age-hardening cold rolled steel plate for deep drawing which comprises up to 0.01% carbon, 0.08 to 0.6% chromium, 0.05 to 0.4% manganese and 0.009-0.02% nitrogen and is free from aluminum as an effective component, the balance being iron and inevitable impurities, and (b) a process for preparing age-hardening cold rolled steel plates for deep drawing, which comprises subjecting a steel comprising up to 0.01% carbon, 0.08 to 0.6% chromium, 0.05 to 0.4% manganese and 0.009 to 0.02% nitrogen and being free of aluminum as an effective component, with the balance being iron and inevitable impurities, to hot rolling and cold rolling according to customary procedures, annealing the resulting steel plate at a temperature of 700.degree. C., to the Ac.sub.3 point and cooling the annealed steel plate to room temperature from about 500.degree.-600.degree. C. at a cooling rate of 100.degree. to 500.degree. C. per hour.
In the steel plate (a) of the above prior art, nitrogen and chromium are incorporated in a very low carbon rimmed steel, and a crystal arrangement suitable for deep drawing of the cold rolled steel plate is formed by the combined effects of nitrogen and chromium. The nitrogen content is higher than 0.009%, and the chromium content is changed depending on the nitrogen and carbon contents and it is, for example, 0.15 to 0.25% when the nitrogen content is 0.01%.
When the steel of the present invention (referred to as "the former steel") is compared with the above steel of the prior art (referred to as "the latter steel"), it is apparent that both steels are quite different from each other. More specifically, the former steel is a hot rolled, steel plate which possesses good press-formability and non-aging properties, whereas the latter steel is a cold rolled steel plate with good press-forming processability and age-hardening properties. Accordingly, it is apparent that both the steels are quite different from each other with respect to the steel composition and characteristics and that the technical concept of the present invention is quite different from that of the above prior art, as described as follows.
As mentioned above, the r value is one of the important characteristics of steel which denotes the formability, especially the deep drawability of the steel. The value can be improved only in the cold rolled steel plate. In the case of a hot rolled steel plate, however, no method has yet been found which improves this value found. Accordingly, another characteristic except value, such as yield point, elongation, and the like, are also important for hot rolled steel plates.
In the automobile industries where large quantities of hot rolled steel plates are used, steel objects of extremely complicated shapes are not required. In order to satisfy these shape requirements, research has been conducted for methods by which hot rolled steel plates possessing very low yield point, properties very high total elongation properites and non-aging properities can be produced. Some of these hot rolled steel plates are now in practical use. One of the current proposals for improving the deep drawability of hot rolled steel plates has been disclosed in, Japanese Patent Application No. 49145/67 (Japanese Patent Publication No. 13/74). As described and illustrated in the claim the specification and the accompanying drawings of the application, the invention resides in a process for the preparation of hot rolled steel plates which possess non-aging properties for the further for cold press forming of the hot rolled steel plates being composed of an aluminum killed low carbon steel comprising up to 0.12% by weight of C, 0.15 to 0.60% by weight of Mn, 0.01 to 0.15% by weight of sol. Al and 0.0040 to 0.0100% by weight of N with the balance being iron and inevitable impurities, said process being characterized by measuring the sol. Al content in the steel, and adjusting the temperature for coiling the hot rolled steel plate to a temperature corresponding to the measured sol. Al content in a region ABCD in FIG. 5 or reheating the hot rolled steel at a temperature of 600.degree. to 700.degree. C., for at least 1 hour after coiling, whereby N in the steel plate is precipitated in the form of AlN and precipitation of large carbides is reduced as much as possible.
According to the disclosure prior art reference, in a hot rolled low carbon aluminum killed steel plate, the AlN precipitation range which provides the highest cold press-formability and non-aging properties is set by adjusting the sol. Al content in the steel and the coiling temperature. The yield point, however, is not included in the specification of the reference. The lowest tensile strength illustrated is about 35 kg/mm.sup.2, and it is estimated that the yield point of a hot rolled steel plate having such a tensile strength may be in the range of 23 to 25 Kg/mm.sup.2. Accordingly, it is believed that the hot rolled steel plate derived from the prior art procedure fails to completely satisfy the requirements for steel plates of this type.